Remove "metamta.*.subject-prefix" options

Summary:
In ~2012, the first of these options was added because someone who hates dogs and works at Asana also hated [Differential] in the subject line. The use case there was actually removing the text, not changing it, but I made the prefix editable since it seemed like slightly less of a one-off.

These options are among the dumbest and most useless config options we have and very rarely used, see T11760. A very small number of instances have configured one of these options.

Newer applications stopped providing these options and no one has complained.

You can get the same effect with translation.override. Although I'm not sure we'll keep that around forever, it's a reasonable replacement today. I'll call out an example in the changelog to help installs that want to preserve this option.

If we did want to provide this, it should just be in Applications → Settings for each application, but I think it's wildly-low-value and "hack via translations" or "local patch" are entirely reasonable if you really want to change these strings.

Test Plan: Grepped for subject-prefix.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19993